There are numerous types of films and materials which are generally being described as "degradable". These materials include polymers which are photodegradable, chemically degradable, biodegradable or semi-biodegradable like some starch-additive-and-polymer formulations.
While conventional polyethylene backsheets and the starch-filled polyethylene backsheets may be broken down into small pieces by mechanical means (i.e., by shredding or mechanical actions inside the rotating composter), they are not compostable. The conventional polyethylene backsheets and the starch-filled polyethylene backsheets are not biodegradable. Biodegradation (or biodecomposition) of the starch-filled plastic films by microorganisms is a very slow process. Microorganisms can only attack the starch granules on the very top surface of a starch-filled plastic film. The remaindering plastic film is not biodegradable.
Commercial starch-filled plastic films contain about 6 to 10% by weight of starch, a pro-oxidant (or pro-degradant) and an accelerator. The pro-oxidant is an oxidation catalyst, which aids in the thermal or photodegradation of the film. Suitable pro-oxidants include transition metal salts including, for example, manganese stearate or a blend of manganese stearate (thermal) and iron stearate (photo). The accelerators typically include an unstable, unsaturated 1,4-diene rubber and a biodegradable filler such as starch. The base resin of the starch-filled films is a stable, non-biodegradable resin such as polyethylene, polypropylene or polystyrene.
For example, the Griffin U.S. Pat. No. 4,983,651 and PCT WO88/09354 relate to a starch-based antioxidant/pro-oxidant resin blend. The presence of the antioxidant and the pro-oxidant give an "induction period" before a sharp loss in physical strength of the resin occurs. The starch-filled plastic films contain a phenolic-type antioxidant such as 4-methyl-1,6-di(2-hydroxy-3-tertbutyl-5-methylbenzyl) phenol which is normally used for stabilizing most low density polyethylene materials. The total amount of antioxidant in the starch-filled polyolefin resin blends is the amount presented in the polyolefin base resin plus the amount in the carrier resins of the additives and also in the unsaturated rubber. No additional antioxidant is added to the starch-filled resin blends. The amount of antioxidant in the resin blend is partially consumed during the film extrusion processes and the remainder is gradually consumed during aging. The remaining antioxidant is used to maintain a "shelf-life" (i.e., an induction period). However, this antioxidant/pro-oxidant system does not have well controlled shelf-life or rate of degradation.
The Gilead et al., U.S. Pat. Nos. 4,519,161 and 4,461,853 disclose "time-controlled" photodegradable mulch films which contain a mixture of a photoactivator and a photostabilizer (antioxidant). The photoactivator is an iron, manganese, or cerium complex. The photostabilizer (or ultraviolet light stabilizer) is a non-ionic organosoluble antioxidant such as a nickel or cobalt complex. These transition metal complexes can be selected from the group consisting of a dithiocarbamate, dithiophosphate, xanthate, cyclic phosphate, oxime, benzthiazole and benimidazole. The metal (iron, nickel or cobalt) complexes are derived from an alkyl-dithiocarbamate such as dibutyldithiocarbamates. The Gilead et al. '161 and '853 disclose the use of conventional thermal antioxidants/stabilizers such as 1,1,2-tris-4-(2-tert-butyl-4-hydroxy-5-methyl)-phenylpropane and BHT (butylated hydroxy toluene or di-tert-butyl-p-cresol). As described in the Gilead et al. '161 and '853 patents, the optional antioxidant is used to provide additional stability during film or fiber forming operation but the antioxidant itself plays no part in determining the embrittlement time.
In the Gilead et al. '161 and '853 patents, the nickel dibutyldithiocabamate (NBC or NDBC) UV light stabilizer functions both by decomposing hydroperoxides and by being a strong UV absorber (or photostabilizer). The photo-stabilizer provides a "controllable induction period" during which the properties of the polymer do not change. The controllable induction period is followed by rapid deterioration of mechanical properties due to "photo-oxidation" triggered by the photoactivator after exposed to UV light. The length of the induction period can be controlled by the relative concentration of two metal complexes (i.e., nickel and iron complexes). However, the UV light source (i.e., the sunlight) is not controllable. The amount of UV light received by the plastic mulch film can be affected by a combination of factors such as seasons, geographical locations, weather conditions, type of crop, etc. Therefore, the induction period is not controllable.
These "starch-based antioxidant/pro-oxidant" polymers and the "antioxidant/photoactivator" polymers can not be used to produce compostable films suitable for diaper backsheets and yard waste bags. The plastic films used in diaper backsheets and yard waste bags do not receive UV radiation to cause the film to breakdown into fragments small enough to pass an initial screening in a composter or windrow and the molecular weights of the polyolefin resins do not decrease enough (i.e., under 1,000) to be biodegradable. Non-biodegradable, "plastic dust" is not desirable in the final compost product.
There is a need for reducing the amount of non-compostable materials in disposable articles. There is a further need to produce compostable films suitable for such uses as backsheets in absorbent articles and yard waste bag. There is a particular need to replace the non-compostable backsheet in absorbent articles with liquid impervious films comprised of compostable materials.
It is an object of the present invention to provide polymer compositions useful for compostable films. The compostable compositions provided by this invention have excellent "long-term shelf-life" under "dry" conditions during melt processing, product storage and end use periods, and can be disintegrated quickly under the "wet" composting conditions. According to the present invention, the molecular weights of the compostable polymer composition decrease continuously both during and after composting such that the compostable compositions can ultimately be biodegraded in the presence of micro-organisms.
Another object of the present invention is to provide compostable, liquid impervious thermoplastic films which are suitable for use as backsheets in absorbent articles such as diapers, sanitary napkins, pantiliners, and the like, which are adapted for absorbing various bodily fluids.
Another object of the present invention is to provide a compostable film which is suitable for use as film backings for diaper fastening tab, release tapes, and frontal target strip in many disposable products.
It is also an object of the present invention to provide a compostable film which is suitable for a variety of product applications such as plastic yard waste bags, plastic garbage bags, agricultural mulch film, heat-sealable packaging films, and other such disposable products.